1. Field of the Invention
The present invention relates to a technique to ensure improvement in stability of withstand voltage and an increase of withstand voltage in a semiconductor device, especially in a high breakdown voltage semiconductor device.
2. Description of the Background Art
In order to drive two power switching devices (MOSFET, IGBT or the like) of high side and low side, for example in a half-bridge type inverter, a power-device driving apparatus having a high side (high potential island) driving circuit for driving the high side power switching device and a low side driving circuit for driving the low side power switching device. Since the high side circuit is operated with its potential in a floating state with respect to the ground potential, such a power-device driving apparatus is provided with a level shifting circuit for transmitting a driving signal to the high side driving circuit. A general-type level shifting circuit consists of a high breakdown voltage switching element such as a MOSFET which is driven by a driving signal and a level shifting resistor connected thereto in series (see FIG. 2 discussed later). A voltage drop in the level shifting resistor is transmitted to the high side driving circuit as the driving signal. The high breakdown voltage switching element needs a stable and high withstand voltage for preventing a breakage in the power-device driving apparatus or generation of wrong signals in the level shifting circuit.
As a technique for improvement in stability and increase of withstand voltage of the high breakdown voltage switching element such as a diode, well known are, for example, a method of forming a plurality of field plates in a floating state (hereinafter, referred to simply as “floating field plates”) on a semiconductor substrate with an insulating film interposed therebetween to uniformize an electric field distribution in a substrate surface (shown in e.g., Japanese Patent Application Laid Open Gazette No. 10-341018 (1998)) and a method of using a RESURF structure as a semiconductor element structure to accelerate depletion in a substrate (shown in U.S. Pat. No. 4,292,642).
When a high voltage is applied to a high breakdown voltage semiconductor device in a cut-off state (OFF state), the high breakdown voltage semiconductor device keeps the voltage. At that time, if an electric field concentration (electric field peak) locally occurs in a semiconductor substrate on which the device is formed, it becomes easier to cause a breakdown in a pn junction or a breakage in an insulating film at that local portion and this causes deterioration of withstand voltage characteristic. In a case where the high breakdown voltage semiconductor device is an n-channel type MOSFET having a RESURF structure, for example, an electric field peak is likely to occur near a drain-side n layer in a junction depth between an n− layer formed on an upper portion of the semiconductor substrate and a p− substrate therebelow and a substrate of the semiconductor substrate below an electrode and an edge portion of the field plate (discussed in detail later).
In an actual use of the high breakdown voltage semiconductor device, its upper surface is covered with an overcoat insulating film or an epoxy resin for assembly. When a high voltage is applied between a drain and a source of the MOSFET in an OFF state and an electric field peak occurs inside thereof, for example, this causes polarization of the overcoat insulating film or the epoxy resin for assembly. Electric charges generated by the polarization are kept for a certain period of time after the application of the high voltage is finished. Then, when a high voltage is next applied between the drain and the source of the MOSFET, with the electric charges, extension of a depletion layer is suppressed locally (especially, near a surface of a silicon substrate). In the local portion where extension of the depletion layer is suppressed, the peak of electric field becomes higher. When the peak reaches a breakdown critical electric field in the silicon surface, this causes a decrease or variation in withstand voltage, or a breakage of the semiconductor device under certain circumstances.